Optically pumped lasers convert pump energy at a pump wavelength into a coherent electromagnetic wave (“laser energy”) at a second wavelength either as a free running laser oscillator, or under control of an input signal as a laser amplifier. A primary goal of laser operation is maintaining laser oscillator action. This requires a threshold pump level and an upper limit on the degree of thermally induced distortion of the optical properties of the laser gain material. It is a further goal to maintain the laser oscillator action with good beam quality. This requires a yet more complete removal of sources of optical distortion of the gain material.
Pump energy that is absorbed and not converted into laser energy becomes heat and must be removed from the laser. The heat generated in connection with the laser action typically becomes larger as the laser power is increased. Eventually, with increasing laser power, the distortion caused by this increasing amount of heat degrades beam quality and will, at sufficiently high power levels, reduce or eliminate the laser oscillator action entirely. This is the primary mechanism that limits the scaling of laser oscillators and amplifiers to high power.
In a conventional pumped laser configuration, the gain medium is configured as a rod, in which the length of the rod is larger than the diameter of the rod. Typically, pumped lasers are surface cooled, that is, heat is conducted away from the surface of the pumped laser gain medium, where side heat removal is relied upon to achieve the heat transfer. Surface cooling results in thermal gradients within the gain medium with a temperature increasing with decreasing radial position relative to the center of the rod. These thermal gradients are necessary for removing the heat generated in the laser material, but introduce several disadvantages from a materials standpoint and, more importantly, increase laser distortion. Therefore, simple surface cooling of a gain medium is inadequate for high-power lasers, such as those currently under consideration for transforming sunlight in space to highly coherent light and transmitting that light safely to Earth and other locations in space at high power levels. These problems occur to some degree in virtually all solid state lasers operating at average powers above a few watts.
More recently, it has been found that a high-power gain medium may be more adequately cooled by having the laser gain media sandwiched between pairs of optically transparent materials having high thermal conductivity. This provides an increased level of cooling of the interior of the laser gain media and reduces the distortion caused by over heating of the laser gain media. However, for optimum high-power laser performance, such as sought for lasers proposed for transferring energy from space, or other applications requiring less average power, the gain media still produces unacceptable distortion, even with the sandwiched high thermal conductivity material.
It is desired to provide a solid state laser medium capable of providing adequate cooling to the gain medium at high-power operation in order to reduce or eliminate thermal induced optical distortion.